1. Field of the Invention
The present invention relates to a multiple-access communications system which has particular, but not exclusive, applications in mobile radio dynamic channel assignment systems, local area networks (LANS) and satellite links. For convenience of description, the invention will be described with reference to mobile radio dynamic channel assignment (trunking) systems but it is to be understood that the same theory applies to other multiple-access communications systems.
2. Description of the Prior Art
Trunking systems are characterised by the problems of many users attempting to gain access to them at the same time. These attempts for access (which can be thought of as requests for service) can clash and in the absence of any form of control can produce an unstable situation where the requests for service arrive at a faster rate than the system can service them. The requests for service are transmitted to a central system control computer hereinafter referred to as a system controller via a signalling channel and the system controller allocates the speech channels according to some predetermined criteria. In the simplest case of a single channel trunking system then the single channel has to be used for signalling and speech.
In order to mitigate these problems controlled multi-access protocols are used to discipline users trying to gain access. Also the throughput, that is the number of successfully serviced requests per unit time, of the system can be increased.
N. Abramson "The Aloha System--Another Alternative for Computer Communications" AFIPS Conference Proceedings 1970 Fall Joint Computer Conference, 37, 281-285 proposed one of the first multi-access protocols termed "Pure Aloha". With this protocol, users transmit a request and wait for some form of acknowledgement of their request from the system controller; if no acknowledgement is heard users wait a random time before re-trying.
The throughput of "Pure Aloha" was doubled by a modified protocol, termed "Slotted Aloha", which allows users to transmit requests only within discrete time slots, each request occupying one complete time slot. In spite of this improved throughput, "Slotted Aloha" nevertheless has practical disadvantages.
With the objective of overcoming these disadvantages, "Slotted Aloha" was extended by a protocol, termed "Framed Aloha", which is disclosed in British Patent Specification No. 2063011A. In Framed Aloha a synchronisation message, termed "Aloha Now", is transmitted by the system controller on the signalling channel at intervals indicating that the immediately following n time slots are available for users to transmit requests (either new requests or re-transmissions after unsuccessful requests) to the system controller via the signalling channel. The number of time slots n is a constant determined at the system design stage. With this protocol the requests are contained within known time frames, simplifying the system control strategy. However a drawback to having a fixed number n of time slots is that it does not take into account the variation in the number of requests between a quiet period and a busy period and this can lead to an unstable situation. Also in a quiet period when perhaps a single request is received in the first time slot then the caller has to wait until another (n-1) slots have elapsed before the system controller allocates a speech channel to the caller.
An attempt to match the number of time slots available with the number of requests is disclosed in British Patent Specification No. 2069799 and is termed "Dynamic Framelength Aloha". This protocol includes means for dynamically controlling the frame length, that is updating the number n of time slots on a frame-by-frame basis. The number n is calculated by observing the events in the previous frame such as the number of garbled (or clashed), empty and successful slots and also from an estimate of the call arrival rate. By using feedback control in this way stability is achieved and additionally the access time (the time delay between a user wishing to make a request, and the request being acknowledged) is reduced. Nevertheless there is still a delay between request acknowledgement and speech channel allocation. An experimental trunking system using Dynamic Framelength Aloha is disclosed in an article "A High Efficiency Mobile Radio Trunking System--Performance Evaluation" by C. K. Davis, P. J. Stein, J. de Jager and R. Postema, published in the IEE Conference on Communications Equipment and Systems--Communications '82 Birmingham April 1982, pages 131 to 137.
Although Dynamic Framelength Aloha is a practical system giving a good performance, there is still a demand to make better use of the time available, particularly in busy periods. One way of doing this would be to increase the signalling rate to say 2400 bits/second instead of 1200 bits/second mentioned in the above article and to reduce the length of each time slot from 100 ms to 60 ms. However this may lead to an undesirable increase in the cost and complexity of the system hardware.
An object of the present invention is to make better use of the time available by means of a different protocol to those described so far.